In general, inverters for driving brushless motors have six switches. After a defect, each switch may in principle have two different characteristics: switched off, that is say blocked in the open switch position, or switched on, that is to say blocked in the closed switch position. A switched-on defective switch is also referred to in the art as a short. Particularly in safety-relevant applications, it is then important to determine very quickly what type of defect is present, in order to continue to operate the electric motor in an emergency mode, or to switch it off immediately.
Frequently, current sensors are used by means of which the current in the individual motor phases or the total current can be measured. Although current sensors make it possible to distinguish between switched-on defective switches and switched-off defective switches, and thus to determine the type of defect, the costs for the current sensors must, however, be taken into account. Alternatively, the electric motor can always be switched off when a switch defect occurs. No emergency mode is then possible.
DE 11 2004 002 642 T5, which is incorporated by reference, discloses a plausibility check for an electrical three-phase system. In order to identify defects reliably, the phase currents are measured. In order to avoid confusion between the phase currents, a current vector is also observed. Comparatively expensive current sensors are therefore required to carry out the abovementioned method.